1. Field of the Invention
The present invention relates to a heat treatment method and an apparatus for a substrate, which improve characteristics of the substance by applying light thereto and are preferably used, in particular, when an amorphous silicon film is formed into a polycrystalline film upon manufacturing a semiconductor substrate and a thin-film transistor (TFT) or the like to be used for a liquid crystal display apparatus.
2. Description of the Background Art
A small-size, high precision active-matrix-type liquid crystal display (LCD) panel, which uses a polycrystalline silicon thin-film transistor (TFT) as its switching element, has been developed for use in a high-resolution display. The application of a polysilicon TFT to an active element of an LCD makes it possible to form a pixel array unit and a driving array unit on the same transparent insulating substrate in the same process; thus, the resulting advantage is that processes, such as wire bonding and packaging of driving ICs, can be eliminated.
The polycrystalline silicon TFT technique is a technique for assembling a thin-film transistor (TFT) by using a semiconductor thin film (with a thickness of approximately several tens to several hundreds nm) formed on a substrate having an insulating surface. When the TFT is applied as a switching element of an LCD, a driving frequency of not less than several hundreds kHz is required for the driving circuit, with the result that in order to form a driving circuit, a TFT which utilizes a polycrystal silicon film (polysilicon film) as an active layer is required.
Conventionally, with respect to a technique for manufacturing a polysilicon film at low costs, there has been proposed a heat treatment method which applies light to an amorphous silicon thin film or a material that has been made amorphous by ion-injecting silicon into a polycrystalline silicon thin film so as to poly-crystallize the film. For example, an SiO2 (silicon oxide) film is formed as an insulating film serving as a base film on a glass substrate, and an amorphous silicon film is formed on the SiO2 film by using a reduced-pressure CVD (Chemical Vapor Deposition) method, and this is subjected to heat treatments such as a laser annealing process, a solid phase growth process and a lamp annealing process so that the amorphous silicon film is crystallized to form a polysilicon film.
By doping this polysilicon film with a predetermined amount of donor and acceptor, a channel formation area, a drain area and the like are formed. Moreover, by forming a gate insulating film, a gate electrode, a source electrode, a drain electrode, an interlayer insulating film, etc. thereon, a thin-film transistor is formed, and a semiconductor device including such a thin-film transistor is manufactured.
In the case when a crystallized polysilicon film is doped with an impurity such as phosphor and boron, a light irradiation process is applied thereto so as to activate the impurity injected through the doping process and modify the interface of the film.
However, the following problems have been raised in the conventional example having such an arrangement.
In the above-mentioned conventional light-irradiation heat treatment system, the crystallization of the polysilicon film is greatly influenced by the degree of heating in the heat treatment process (for example, laser irradiation energy, laser wavelength or the like in the laser annealing process). Consequently, the transistor characteristics of the resulting thin-film transistor change greatly in response to the degree of heating sensitively. In other words, in the case when a laser annealing process is carried out by setting a specific laser irradiation energy, if this energy is varied due to deviations in the characteristics of the irradiation apparatus and a change in the operational environment such as the operation temperature, the deviations in crystallization of the polysilicon film formed through the laser annealing process become greater, with the result that the threshold voltage in the thin-film transistor tends to deviate from the designed value greatly in a range of approximately several volts.
For this reason, it is not possible to obtain a thin-film transistor, etc. having desired transistor characteristics stably unless the laser irradiation energy is controlled with extremely high precision upon practical use at the time of the laser annealing process. In other words, the application of standard laser irradiation apparatuses, etc. currently used widely makes it difficult to manufacture a semiconductor device including a thin-film transistor, etc. having desired transistor characteristics, and tends to cause malfunctions in the resulting semiconductor device and a higher rate of defective products.
Moreover, in the case of a lamp annealing process using a halogen lamp, the light-emitting energy of a light-emitting area to be absorbed in a silicon film is not sufficiently high, and it is necessary to provide a means for increasing light intensity or for making the irradiation time longer in order to carry out a sufficient process. However, such a method increases not only the temperature of the silicon film on the glass substrate but also the temperature of the glass substrate to a level exceeding the necessary temperature, resulting in a warp and a distortion in the substrate.
In the above-mentioned conventional light irradiation heat treatment system, in order to meet the demands for large-size liquid crystal glass TFT substrates, a technique has been proposed in which a light irradiation process is carried out on the entire surface of a substrate by allowing a large-size substrate to pass through a light irradiation area of a light source, by relatively shifting the light source and the large-size substrate. The advantage of this arrangement is that processes are continuously carried out on the large-size substrate. Moreover, a heat treatment apparatus, which preliminarily heats the substrate by using a pre-heating part before the light irradiation of the light sources, has been proposed. Here, with respect to the pre-heating part used for the above-mentioned heat treatment apparatus, a light source such as a lamp is used. With respect to the light source, a flash lamp or the like is used so that flash light is applied to the surface of the substrate to raise only the temperature of the silicon film in a short time.
However, in the case when the flash light is applied so as to raise the temperature of the surface of the substrate, the flash light and heat radiation that have passed through the substrate tend to cause burning of a filament and degradation in the lamp of the pre-heating part.